In general, a conventional weldable coated steel sheet is a steel sheet manufactured by coating organic solvent type resin containing metal powder, and aqueous chrome-free treatment must be applied to the steel sheet in order to secure adhesion with the steel sheet and corrosion resistance. The chrome-free coating formed on the steel sheet is less than 1 μm thick, but cannot contain conductive pigments such as metal powder, conductive ceramic, graphene and so on, in order to secure corrosion resistance, and hence, is deteriorated in weldability.
In the meantime, a coated steel sheet for shielding electromagnetic waves in order to enhance conductivity of a surface-coated layer is manufactured when aqueous paint composition containing metal powder is coated on the steel sheet. Such an aqueous paint composition containing metal powder is superior in adhesion with the steel sheet, but is vulnerable to corrosion resistance, thus it is difficult to be utilized as a steel sheet for construction materials which require high-degree corrosion resistance.
A surface coated steel sheet which is coated with UV-curing paint composition has superior appearance and scratch-resistance, but is difficult to be welded because a thick surface-coated layer is formed on the surface of the steel sheet. Particularly, because a curing reaction of the UV-curing paint composition occurs by irradiation of ultraviolet rays, it is difficult to use opaque metal powder or pigments into the UV-curing paint composition.
Therefore, in order to use the surface coated steel sheet coated with the UV-curing paint composition, which has superior appearance and scratch-resistance, in the field of the construction field, it is necessary to develop a coated steel sheet which enhance coating adhesion with the steel sheet, secure corrosion resistance and has superior weldability.
Accordingly, the present invention has been made in an effort to solve the above-mentioned problems occurring in the prior arts, and it is an object of the present invention to provide a coated steel sheet having superior weldability, scratch-resistance, and corrosion-resistance properties, and a method of manufacturing the same.